The present embodiments relate to digital sound processing, and are more particularly directed to digital loudness compensation.
For decades it has been understood that the human listener's perception of loudness varies with frequency when sound pressure (i.e., volume) is held constant. Specifically, for a fixed sound pressure, bass frequencies are perceived at a lower loudness than are treble frequencies. As a result, various sound processing approaches have arisen to process pre-recorded sound so that it is played back in a modified manner.
One sound processing approach in the art amplifies relatively low audible frequency signals to a greater volume, thereby providing a human with the perception that all frequencies are being heard at a like sound pressure. This approach has been referred to in the art as “loudness compensation.” Loudness compensation is typically performed with reference to various loudness versus frequency curves that have been created by persons skilled in the art and that present what the creator of the curves believes is an ideal modification to the sound based on the frequency of the sound. Such curves, for example, have been provided in works by S. S. Stevens, who attempted to improve upon the so-called Fletcher-Munson curves produced by G. H. Fletcher and W. A. Munson.
Another sound processing approach in the art is based on the difference between the sound level at the time the sound was recorded versus the sound level at which the sound is played back. Thus, this approach focuses not on the difference in sound frequency but instead on the difference between the recording and playback volumes, and this approach is sometimes referred to in the art as “differential loudness compensation.” Differential loudness compensation also consults various curves, where by way of example a set of such curves are shown in FIG. 1. In the example of FIG. 1, a sound recording is made at 85 dB, and eight plots 101 through 108 are shown corresponding to gain adjustments that are recommended to be made to the sound when presented at playback levels different than 85 dB. More particularly, plot 101 corresponds to the gain adjustments to be made for a playback at 45 dB, plot 102 corresponds to the gain adjustments to be made for a playback at 50 dB, and so forth for the remaining plots at 5 dB increments up to plot 108 which corresponds to the gain adjustments to be made for a playback at 75 dB. A similar set of curves may be generated for different recording sound pressure levels. However, both the gains and the shapes of the curves may vary somewhat for the various recording levels. In any event, differential loudness compensation has particular application where music has been played and recorded live, and there is a desire for a person listening to a subsequent playback of the recording (at a different volume) to experience the music as if the person were attending the original live performance.
The actual circuit implementations of the sound compensation methodologies described above also have varied in the art based on various criteria, and many of these implementations have limitations. For example, there is often the question of whether an implementation should be directed to loudness compensation or differential loudness compensation. As another example, there often are considerations of the physical limitations, complexity, and costs of audio systems and compensation systems. As still another example, often there is correction only at very low audible frequencies, such as through the use of a bandpass filter centered at those frequencies. However, such an approach may not be workable in a relatively inexpensive audio system that has poor (or no) response at such low frequencies. As a result, some of the prior approaches may be implemented at slightly higher frequencies, but in these alternatives there is the possibility that higher frequency signals, such as vocal frequencies, may be modified in a manner that is undesirable or inconsistent with the human ear's original failure to accurately perceive the lower frequencies. As yet another example, considerations arise regarding the media to be reproduced as well as listener preferences. As still another example, differential loudness compensation may not be feasible or may be difficult for some types of pre-recorded sounds because typically the sound pressure of the level of the sound at the time of recording is not directly encoded or otherwise provided along with the recording, where instead there typically is only some electrical, mechanical, or numerical representation of the sound itself.
By way of further background, U.S. Pat. No. 4,739,514 (“the '514 patent”) issued on Apr. 19, 1988 to Short et al., is entitled “Automatic Dynamic Equalizing,” and is hereby incorporated herein by reference. The '514 patent illustrates in its FIG. 1 a sound processing circuit where an audio input signal (11), previously having been adjusted by a volume control, is coupled both to a signal combiner (14) and a compressor (15). The compressor adjusts the gain of the audio input signal in response to the energy of that signal. More particularly, the compressor performs what is urged as a 2:1 compression ratio based on the energy of the audio signal so that for every increase in 2 dB of the audio input signal, the gain is decreased by 1 dB. Further, the gain-adjusted signal is then passed through a bandpass filter (16) centered about a relatively low frequency on the order of 55–70 Hz. The resulting signals are shown in FIG. 2 of the '514 patent, from which one skilled in the art will appreciate that the amplified and filtered signals are centered between 50 and 60 Hz, and a lesser gain is applied by the compressor in response to ascending levels of input energy.
While the '514 patent as well as other approaches in the art may provide acceptable sound processing in some circumstances, the present inventor has recognized various additional drawbacks in such approaches when implemented in certain systems. For example, the approach of the '514 patent explicitly avoids the use of a volume control as a contributor to its equalization approach, while as shown later the present inventor has found the use of a volume control in a loudness compensation circuit to be beneficial, particularly in a digital implementation. As another example, the '514 patent focuses primarily on a compression ratio of 2:1 (although it suggests that such a ratio need not be restricted thereto), where such a restriction may not provide adequate flexibility given various system considerations or user preferences. As a final example, the previous approaches are often relatively complex or sacrifice quality and/or the ability to configure the sound processing methodology, where such aspects are likely highly desirable in certain sound reproduction systems.
In view of the above, there arises a need to address the drawbacks of the prior art sound compensation systems, and this need is explored in connection with the preferred embodiments described below.